Theory of exchange narrowing in low-dimensional correlated spin systems

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Abstract
Linear response theory is extended to magnetic resonance absorption (EPR) in low‐dimensional networks with strong isotropic exchange J0 between adjacent spins. Previous high temperature results are generalized to the regime kT≳J0≫h/ω0, where ω0 is the Larmor frequency, in which spin correlations leading to temperature dependent EPR linewidths can be included by taking thermal averages over the dominant isotropic exchange interaction. Mori’s generalized Langevin equation gives the absorption profile I (ω−ω0) by a memory function which explicitly treats the slow decay of spin correlations in low‐dimensional systems, thus identifying diffuse contributions to four‐spin correlation functions without decoupling. The theory is illustrated for two‐dimensional antiferromagnetic MnCl−24 layers with dipolar broadening, where it yields the general angular dependence of the EPR linewidth. Approximate computations of long‐time cutoffs, their experimental determination, and the relation of the general theory to previous RPA treatments are discussed.